1. Field of the Invention
The present invention relates to a valve device and valve control method. The valve is mounted in an engine system and reduces the concentration of exhaust gases by recirculating engine exhaust gases to the engine combustion chamber.
2. Description of the Related Art
FIG. 1 shows an engine system mounting a conventional Valve device. In the figure, reference numeral 1 denotes an air cleaner which removes particulate matter contained in external air and transfers such air to an air intake 3. 2 is an injector which injects fuel (for example gasoline) into the air intake 3. 3 is an air intake which supplies a gaseous mixture and fuel to the engine 5. 4 is a throttle valve which regulates the amount of the gaseous mixture supplied to the engine 5. 5 is an engine of an automobile which transmits drive force to the drive system by the combustion of the gaseous mixture. 5a is a combustion chamber of the engine 5, 5b is an intake valve closing the communication of the combustion chamber 5a with the air intake 3, 5c is an exhaust valve which closes the communication of the combustion chamber 5a and the exhaust outlet 6, 5d is a piston which displaces vertically in the combustion chamber 5a.
6 is an exhaust outlet which exhausts a gaseous mixture (exhaust gas) which has been combusted in the engine 5. 7 is a purification device which allows emission of exhaust gases to the atmosphere after their purification. 8 is a re-circulation pipe which circulates a part of the exhaust gases exhausted from the engine 5 to the engine combustion chamber 5a. 9 is an EGR valve which is disposed in the re-circulation pipe 8 and which is a valve device which regulates an re-circulated amount of exhaust gases. 10 is a control unit which controls the aperture of the EGR valve 9 in response to an operational state of the vehicle.
FIG. 2 is a cross sectional view of a conventional valve device (EGR valve). In the FIG., 11 is a housing of an EGR valve 9,12 is a through passage connected with the re-circulation pipe 8 on the exhaust outlet 6 side. 13 is a through passage connected with the re-circulation pipe 8 on the air intake 3 side. 14 is a valve disposed between the through passage 12 and the through passage 13. 14a is an abutting member which abuts with the valve 14. 15 is a valve rod which supports the valve 14. 16 is a spring support member, 17 is a spring which compresses the valve rod 15 upwardly. 18 is a step motor which displaces the drive rod 19 vertically when regulating the aperture of the valve 14. 19 is a drive rod which displaces a valve rod 15 upwardly together with the rotation of the step motor 18.
The operation of the invention will be described below.
When the engine 5 receives a gaseous mixture of fuel and air from the air intake 3, drive force is transmitted to the drive system by the reciprocal motion of the piston 5d by combustion of the gaseous mixture. Exhaust gases which result from the combustion of the gaseous mixture are output to the exhaust outlet 6 from the combustion chamber 5a.
The majority of the exhaust gases are emitted into the atmosphere after purification by the purification device 7. In order to reduce the concentration of the exhaust gases, a part of the exhaust gases is re-circulated to the combustion chamber 5a of the engine 5 through the re-circulation pipe 8.
The amount of re-circulation of exhaust gases re-circulated to the combustion chamber 5a of the engine 5 is regulated by the EGR valve 9 disposed in the re-circulation pipe 8 and depends on an operational condition of the vehicle.
Hereafter control of the re-circulated amount of exhaust gases by the EGR valve 9 will be described.
Firstly when the engine is stopped, the valve rod 15 and the drive rod 19 are in an opposed state. Although the valve rod 15 does not receive a downward depressing force from the drive rod 19, since an upward force is applied by the spring 17, the valve rod 15 abuts with the abutting member 14a and the re-circulation of the exhaust gases is stopped.
On the other hand, when the engine 5 is started in order to regulate an amount of engine exhaust gases balanced with the vehicle operational condition, the control unit 10 controls the aperture of the valve 14 in the EGR valve 9 by outputting a valve lift control signal (a valve signal which commands the opening or closing of the valve 14) to the EGR valve 9 based on the temperature of the engine coolant, the engine rotation speed, the injection pump aperture and the like.
For example, when the EGR valve 9 receives a pulse signal for opening the valve 14, the coil of the step motor 18 is excited and the step motor is rotated in a direction in which the drive rod 19 is depressed. In order for the step motor 18 to maintain a large drive torque, the excitation mode of the step motor 18 adopts a 2-phase excitation.
In this way, when the drive rod 19 is depressed and abuts with the valve rod 15, the valve rod 15 is depressed downwardly, the valve 14 of the EGR valve 9 is opened and the re-circulation of exhaust gases commences.
When the re-circulation amount of exhaust gases is balanced with the operational conditions of the vehicle, that is to say, when the aperture of the valve 14 equals a target value, a valve lift control signal (a pulse signal which commands the opening of the valve 14) received from the control unit 10 is terminated. When a pulse signal commanding the closure of the valve 14 is repeated, the aperture of the valve 14 approaches a target value and the rotation of the step motor is terminated.
The step motor 18 is required to maintain a fixed aperture in the valve 14 by resisting the pressing force of the spring 17 even when rotation is terminated (hereafter referred to as "not driven"). Thus the coils remain in an excited state (2-phase) with a continuous electricity supply (when the motor is driven, the supply of electricity is interrupted when receiving a pulse signal).
Since the conventional valve device is constructed as above, as the coils of the step motor 18 must be excited even when the step motor 18 is not driven and thus the excitation of the coils requires constant supply of electricity, the calorific value and electricity consumption of the coils is greater when the step motor is not driven than when the step motor is driven. Thus cost increases are incurred by the necessity to provide heat resistance with respect to high calorific values while the step motor is not driven. (In particular, when high-speed operation is required, the coil may be operated at low resistance and thus there is a tendency for temperature differentials between driven and non-driven periods to be great).